The circumstances of the massive fish kill on the Mulkear River in Co Limerick is the opposite of the classic scenario for a fish kill. And it has ruined for local fisheries authorities what up to this week had been a "reasonably good" summer, and has caused great disappointment and upset.
Officers with the Shannon Regional Fisheries Board will have to wait until flood waters drop before they can estimate the size of the fish kill but at least eight miles of prime salmon and trout fishing have been destroyed. The source of the kill is believed to have been agricultural effluent - pig or cattle slurry, or ordinary silage.
The classic scenario for a major fish kill is a pollutant entering a river which has depleted water levels because of sustained warm dry weather. The low water levels increase the effect of the pollutant.
However, in the Mulkear river case the surrounding lands are currently flooded because of the heavy rainfall. The river is full and the flow of the waters over weirs and turbulent stretches has meant higher than normal levels of oxygen in the water. Turbulence allows a river to "trap" more oxygen out of the air.
"Normally, you don't expect a fish kill when you have a lot of high water," said Mr Eamon Cusack, chief officer with the Shannon Regional Fisheries Board. "We've had a reasonably good year in regard to fish kills and we've had good water quality because of the high levels due to the rains."
Fish such as salmon and trout require high water oxygen levels in order to survive. What happens during a fish kill is that micro-organisms feeding off the pollutant use up large amounts of oxygen and reduce the levels below those needed to keep salmon and trout alive. "It's akin to putting of lot of people into a small room and how they would use up the oxygen," said Mr Cusack.
The fact that a full river would experience such a kill means that the pollutant which got into the Mulkear must have been particularly strong. Rivers like the Mulkear would normally have an 80 to 100 per cent oxygen level. Measurements taken at Abington Bridge yesterday, just downstream from the source of the pollution at Pallas Grean, revealed an oxygen level of just 15 per cent.
Fisheries investigators are now waiting for the flood waters in the lands around the source of the pollutant to drop so that they can identify individual drains and try to identify the exact source of the kill.
There have been two other massive fish kills this summer. The Moynalty River in Co Meath was destroyed by chemicals which entered it near Mullagh, Co Cavan following a fire in a chemicals factory on July 28th. Around 100,000 fish were killed as the pollution swept down the Moynalty and into the Blackwater. If it were not for the high waters caused by the wet summer, the fish kill would have extended all the way down the Boyne to the sea, according to Ms Marie Fallon, a fisheries environmental officer with the Eastern Fisheries Board.
The Moynalty is a renowned trout river but was "wiped out" by the kill. A prosecution is likely in the case.
The other massive kill was in the Martin and Shournagh rivers in Co Cork. Again around 100,000 fish died. The kill occurred after pig slurry from a Macroom Farm Mills plant at Grenagh, in north Cork entered the rivers on Friday July 18th.
Nevertheless, this has been a better summer than most and the number of "point source" pollutions has in general been dropping in recent years. "The big worry now is the eutrophication of rivers and lakes through the over-use of fertilisers, and because of sewage treatment plants and detergents," said Mr Cusack.
Farmers, he said, persist in the overuse of fertilisers containing phosphates when the lands they are spreading them on are already bloated with the chemical. The excess phosphates wash off the land and into rivers and lakes where they encourage the growth of algae. The consumption of oxygen by the algae causes the oxygen levels to drop below the levels needed for trout and salmon.
